1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine and, particularly, to a control system which can determine a failure in the air-fuel ratio control system.
2. Description of the Related Art
Japanese Patent Laid-open Publication No. 2010-133418 (JP-'418) discloses an apparatus for determining a failure in an air-fuel ratio sensor disposed in the exhaust system of the engine. According to this apparatus, an air-fuel ratio perturbation control is performed during the engine operation for oscillating the air-fuel ratio with a predetermined frequency, and it is determined that the response characteristic deterioration failure has occurred in the air-fuel ratio sensor if an intensity of the predetermined frequency component contained in the output signal of the air-fuel ratio sensor during execution of the perturbation control, is equal to or less than a determination threshold value.
Japanese Patent Laid-open Publication No. 2011-144754 (JP-'754) discloses an air-fuel ratio control system which can determine an imbalance failure that air-fuel ratios corresponding a plurality of cylinders in the engine differ with each other more greatly than the allowable limit, based on the output signal of the air-fuel ratio sensor disposed in the exhaust system of the engine. According to this system, the air-fuel ratio perturbation control is performed during the engine operation for oscillating the air-fuel ratio with a predetermined frequency, and the imbalance failure is determined using a determination parameter which is obtained during the perturbation control. The determination parameter is calculated by dividing an intensity of the 0.5th-order frequency component contained in the output signal of the air-fuel ratio sensor by an intensity of the predetermined frequency component contained in the output signal of the air-fuel ratio sensor. The 0.5th-order frequency component is a component of a frequency which is half of the frequency corresponding to the engine rotational speed. When the imbalance failure occurs, the 0.5th-order frequency component intensity increases, and a value of the determination parameter increases as the degree of the imbalance failure increases. Accordingly, the imbalance failure can be determined by comparing the determination parameter with a predetermined threshold value.
In the apparatus of JP-'418, the predetermined frequency component is extracted using a band-pass filtering for calculating the intensity of the predetermined frequency component. The 0.5th-order frequency component increases when the imbalance in the air-fuel ratios corresponding to the plurality of cylinders exists, even if the imbalance degree is not so large that the imbalance failure is determined to occur. Therefore, if the predetermined frequency is in the vicinity of the 0.5th-order frequency, it is necessary to make the pass-band width WB of the band-bass filtering for extracting the predetermined frequency component narrower.
In the system of JP-'754, the 0.5th-order frequency component and the predetermined frequency component are extracted with the band-pass filtering for calculating the intensities of the 0.5th-order frequency component and the predetermined frequency component. In the imbalance failure determination, it is preferable to set the predetermined frequency in the vicinity of the 0.5th-order frequency, which requires a narrower pass-band width WB of the band-pass filtering.
FIGS. 11A and 11B show graphs for illustrating a relationship between the pass-band width WB and the transient response characteristic. The solid line L1, the broken line L2, and the dot-and-dash line L3 indicated in FIG. 11A correspond respectively to the solid line L11, the broken line L12, and the dot-and-dash line L13 indicated in FIG. 11B. Specifically, the rise time period, from the time t0 of starting the band-pass filtering to the time the filtered output VOUT reaches the steady output VST, becomes longer as the pass-band width WB becomes narrower. FIG. 12A shows an example of the rising characteristic of the filtered output signal. Further, when the input signal of the band-pass filtering (the air-fuel ratio sensor output signal) steeply changes, the initial oscillation as indicated in FIG. 12B may occur more easily as the pass-band width WB becomes narrower.
As described above, a narrow pass-band width WB of the band-pass filtering may cause some problems. Accordingly, it is desired to improve the S/N of the extracted signal, i.e., the ratio of the extracted signal intensity to the noise (unnecessary frequency components) intensity, using a band-pass filtering of a comparatively wide pass-band width WB.